How to use this image

All examples in this guide use the public image. If you’ve mirrored the repository for your own use (for example, to your Docker Hub namespace), update your commands to reference the mirrored image instead of the public one.

For example:

• Public image: dhi.io/<repository>:<tag>
• Mirrored image: <your-namespace>/dhi-<repository>:<tag>

For the examples, you must first use docker login dhi.io to authenticate to the registry to pull the images.

What's included in this ASP.NET Core image

This Docker Hardened ASP.NET Core image includes the ASP.NET Core runtime in a minimal, security-hardened package:

• ASP.NET Core runtime: Complete runtime for running .NET web applications
• dotnet CLI: Command-line interface for running .NET applications (not building)
• TLS certificates: Pre-installed CA certificates for secure connections
• Container optimizations: Pre-configured for non-root execution on port 8080
• Security hardening: Runtime variants have no shell or package manager for a minimal attack surface

Start an ASP.NET Core instance

Run the following command and replace <tag> with the image variant you want to run:

docker run --rm dhi.io/aspnetcore:<tag> dotnet --info

Common ASP.NET Core use cases

Run an ASP.NET Core application

Run your ASP.NET Core application directly from the container. Note that the default port is 8080 for non-root compatibility:

docker run -p 8080:8080 dhi.io/aspnetcore:<tag> dotnet /app/myapp.dll

Build and run a simple web API

Since DHI ASP.NET Core images include the runtime but not the .NET SDK, use dhi.io/dotnet:<version>-sdk to build or publish applications and dhi.io/aspnetcore:<version> to run them:

# Create a new web API project using DHI's SDK
docker run -v $(pwd):/app -w /app dhi.io/dotnet:8-sdk \
  dotnet new webapi -n MyApi --no-https

# Build the application using DHI's SDK
docker run -v $(pwd):/app -w /app dhi.io/dotnet:8-sdk \
  dotnet publish MyApi -c Release -o ./published

# Run with DHI runtime (note: this is a web API, so it needs proper setup)
docker run -v $(pwd)/published:/app -p 8080:8080 dhi.io/aspnetcore:8 \
  dotnet /app/MyApi.dll

Customize the ASP.NET Core runtime root

Use dhi.io/aspnetcore:<version>-dev when you need the ASP.NET Core runtime image family with a package manager, shell, and root user. This is useful for installing extra OS packages in an intermediate stage while keeping the final image on the matching ASP.NET Core runtime base.

The ASP.NET Core dev variant is not a replacement for the .NET SDK image. Use dhi.io/dotnet:<version>-sdk to compile or publish .NET applications, and use dhi.io/aspnetcore:<version>-dev when you need package-manager access against the same runtime family as the final ASP.NET Core image.

For example:

FROM dhi.io/aspnetcore:8-dev AS aspnet-runtime-root
RUN apt-get update && apt-get install -y --no-install-recommends <package>

FROM dhi.io/dotnet:8-sdk AS builder
WORKDIR /src
COPY . .
RUN dotnet publish -c Release -o /app/publish

FROM dhi.io/aspnetcore:8
WORKDIR /app
COPY --from=builder /app/publish .
COPY --from=aspnet-runtime-root /usr/lib/<library> /usr/lib/<library>
USER 1001
ENTRYPOINT ["dotnet", "MyApp.dll"]

Multi-stage build for production

Use DHI's SDK for building and DHI runtime for production:

# Build stage
FROM dhi.io/dotnet:8-sdk AS builder
WORKDIR /src
COPY ["MyApp.csproj", "."]
RUN dotnet restore
COPY . .
RUN dotnet publish -c Release -o /app/publish

# Runtime stage
FROM dhi.io/aspnetcore:8
WORKDIR /app
COPY --from=builder /app/publish .
USER 1001
ENTRYPOINT ["dotnet", "MyApp.dll"]

Docker Official Images vs Docker Hardened Images

Key differences

Why no shell or package manager?

Docker Hardened Images prioritize security through minimalism:

• Reduced attack surface: Fewer binaries mean fewer potential vulnerabilities
• Immutable infrastructure: Runtime containers shouldn't be modified after deployment
• Compliance ready: Meets strict security requirements for regulated environments

The hardened images intended for runtime don't contain a shell nor any tools for debugging. Common debugging methods for applications built with Docker Hardened Images include:

• Docker Debug⁠ to attach to containers
• Docker's Image Mount feature to mount debugging tools
• Ecosystem-specific debugging approaches

Docker Debug provides a shell, common debugging tools, and lets you install other tools in an ephemeral, writable layer that only exists during the debugging session.

For example, you can use Docker Debug:

docker debug <container-name>

or mount debugging tools with the Image Mount feature:

docker run --rm -it --pid container:my-aspnet-app \\
  --mount=type=image,source=dhi.io/busybox,destination=/dbg,ro \\
  dhi.io/aspnetcore:<tag> /dbg/bin/sh

Image variants

Docker Hardened Images come in different variants depending on their intended use.

Runtime variants are designed to run your application in production. These images are intended to be used either directly or as the FROM image in the final stage of a multi-stage build. These images typically:

• Run as the nonroot user
• Do not include a shell or a package manager
• Contain only the minimal set of libraries needed to run the app

Dev variants include dev in the tag and are intended for package installation or other preparation work in an intermediate stage of a multi-stage Dockerfile. These images typically:

• Run as the root user
• Include a shell and package manager
• Are used to install packages against the matching runtime image family

For ASP.NET Core specifically, use dhi.io/dotnet:<version>-sdk to build or compile applications. Use dhi.io/aspnetcore:<version>-dev when you need shell or package-manager access for the ASP.NET Core runtime image family.

FIPS variants include fips in the variant name and tag. They come in both runtime and build-time variants. These variants use cryptographic modules that have been validated under FIPS 140, a U.S. government standard for secure cryptographic operations. For example, usage of MD5 fails in FIPS variants.

Migrate to a Docker Hardened Image

To migrate your application to a Docker Hardened Image, you must update your Dockerfile. At minimum, you must update the base image in your existing Dockerfile to a Docker Hardened Image. This and a few other common changes are listed in the following table of migration notes:

The following steps outline the general migration process.

1. Find hardened images for your app.

   A hardened image may have several variants. Inspect the image tags and find the image variant that meets your needs. ASP.NET⁠ Core images are available in versions 8, 9, and 10.

2. Update the base image in your Dockerfile.

   Update the base image in your application's Dockerfile to the hardened image you found in the previous step. For framework images, this may be an image tagged as dev when you need to install OS packages or dependencies.

3. For multi-stage Dockerfiles, update the runtime image in your Dockerfile.

   To ensure that your final image is as minimal as possible, you should use a multi-stage build. All stages in your Dockerfile should use a hardened image. Use the DHI .NET SDK image for build stages, use ASP.NET Core dev variants for runtime-family package installation, and use non-dev ASP.NET Core variants for the final runtime stage.

4. Install additional packages

   Docker Hardened Images contain minimal packages in order to reduce the potential attack surface. You may need to install additional packages in your Dockerfile. Inspect the image variants to identify which packages are already installed.

   Only images tagged as dev typically have package managers. You should use a multi-stage Dockerfile to install the packages. For ASP.NET Core, use aspnetcore:<version>-dev to install OS packages against the matching ASP.NET Core runtime image family, then copy any necessary artifacts to the runtime stage that uses a non-dev image.

   For Alpine-based images, you can use apk to install packages. For Debian-based images, you can use apt-get to install packages.

Troubleshoot migration

General debugging

The hardened images intended for runtime don't contain a shell nor any tools for debugging. The recommended method for debugging applications built with Docker Hardened Images is to use Docker Debug⁠ to attach to these containers. Docker Debug provides a shell, common debugging tools, and lets you install other tools in an ephemeral, writable layer that only exists during the debugging session.

Permissions

By default image variants intended for runtime, run as the nonroot user. Ensure that necessary files and directories are accessible to the nonroot user. You may need to copy files to different directories or change permissions so your application running as the nonroot user can access them.

Privileged ports

Non-dev hardened images run as a nonroot user by default. As a result, applications in these images can't bind to privileged ports (below 1024) when running in Kubernetes or in Docker Engine versions older than 20.10. Configure your ASP.NET⁠ Core applications to listen on ports 8080, 8443, or other ports above 1024.

No shell

By default, image variants intended for runtime don't contain a shell. Use dev images in intermediate stages to run shell commands and then copy any necessary artifacts into the runtime stage. In addition, use Docker Debug to debug containers with no shell.

Entry point

Docker Hardened Images may have different entry points than images such as Docker Official Images. Use docker inspect to inspect entry points for Docker Hardened Images and update your Dockerfile if necessary.

ASP.NET⁠ Core-specific considerations

Port configuration

ASP.NET⁠ Core applications in .NET 8+ use the ASPNETCORE_HTTP_PORTS environment variable (default: 8080) instead of the older ASPNETCORE_URLS. For .NET 6/7 applications, you may need to set:

ENV ASPNETCORE_URLS=http://+:8080

Static files and wwwroot

Ensure the wwwroot directory for static files is readable by the nonroot user:

COPY --chown=1001:1001 wwwroot ./wwwroot

Blazor and SignalR applications

Applications using SignalR (including Blazor Server) may require additional environment configuration:

ENV ASPNETCORE_FORWARDEDHEADERS_ENABLED=true